A liquid crystal display employing a mode in which a liquid crystal layer having a twisted nematic liquid crystal therein is interposed between two sheets of polarizing plates orthogonal to each other and an electric field is applied in the vertical direction to a substrate, a so-called TN mode, has been widely known. According to this mode, since the liquid crystal rises up against the substrate in a black state, when observed obliquely, birefringence due to the liquid crystal molecule is generated so that light leakage generates. In order to solve this problem, a mode for preventing this light leakage by using a film comprising liquid crystalline molecules hybrid aligned therein to optically compensate a liquid crystal cell is put into practical use. However, even by using such a film, it is very difficult to completely optically compensate the liquid crystal cell without causing any problem, thereby causing a problem that grayscale inversion in the lower direction of the screen cannot be entirely suppressed.
For the purpose of solving such a problem, there have been proposed and put into practical use a liquid crystal display device employing a so-called in-plane switching (IPS) mode, in which the transversal field are applied to liquid-crystal molecules; a so-called fringe field switching (FFS) mode; or a vertically aligned (VA) mode, having multi domains divided by projections formed in a panel or slit electrodes. Recently, such liquid-crystal displays have been developed as a panel employed not only in monitors but also in TV, and the brightness thereof has been improved remarkably. Consequently, small light leakage generating at opposing corners in a black state while being observed in an oblique direction has come to the surface as a cause of lowering displaying-quality.
In order to improve color tones or viewing angles in a black state, it has been also tried that an optical compensatory material exhibiting a birefringence property is disposed between a liquid-crystal layer and a polarizing plate in an IPS mode display. For example, it is disclosed that a birefringent medium capable of compensating increased or decreased retardation of a tilted liquid crystal layer, having optical axes orthogonal to each other, and disposed between a liquid crystal cell substrate and a polarizing plate, can reduce the coloration in an oblique direction in a white or halftone state (TOKKAI No. hei 9-80424).
There have been proposed a method employing a film formed of a styrene based polymer having an intrinsic negative birefringence or a discotic liquid crystal compound (TOKKAI No. hei 10-54982, TOKKAI No. hei 11-202323 or TOKKAI No. hei 9-292522); a method employing a combination, as an optical compensation film, of a positive birefringent film having an optical axis in a plane and a negative birefringent film having an optical axis in a normal direction (TOKKAI No. hei 11-133408); a method employing a biaxial optical compensation sheet having a λ/2 retardation (TOKKAI No. hei 11-305217); and a method employing a combination of a negative retardation film, as a protective film of a polarizer, and a positive retardation film, as an optical compensation layer disposed on the surface of the protective film (TOKKAI No. hei 10-307291).
Many of the proposed methods are methods to improve viewing angles by counteracting the birefringence of liquid crystal in the cell, and cannot sufficiently prevent light leakage generating while the liquid-crystal displays are observed in oblique direction, or, in other words, the polarizing axes are out of orthogonal alignment. Some of the proposed methods are for reducing such light leakage and, however, even if such methods are employed, it is extremely difficult to optically compensate the liquid-crystal cell perfectly. Known optical compensatory sheets used for an IPS mode liquid-crystal cell are thick since they consists of plural films, and this is disadvantageous for thinning of liquid-crystal displays. Some of the optical compensatory sheets are prepared by stacking stretched films with adhesion layers. The adhesion layers shrink depending on variation of temperature or humidity, and, thus, film peel or warpage is sometimes occurred.
There has been also proposed a method for achieving optical compensation of IPS by employing a combination of retardation films having positive birefringence whose optical axis is present in the normal direction thereof. As a method for preparing such a retardation film, there is known a method for vertically aligning a liquid crystal material and immobilizing the alignment state (see, for example, JPT 2000-514202, TOKKAI No. hei 10-319408, and TOKKAI No. hei 6-331826).
Furthermore, as a method for vertically aligning a rod-like liquid crystalline compound, there are proposed a method for using a vertical alignment layer as an alignment layer; and a method employing a layer of a vertically aligning agent (for example, a quaternary ammonium-substituted silane coupling agent), which is formed on a substrate to be a layer under a layer of a liquid crystalline compound (for example, Koji OKANO, et al., Ekisho: Oyo-hen (Liquid Crystals: Applied Edition), published by Baifukan Co. Ltd. (1985), page 61; and Shohei NAEMURA, Appl. Phys. Lett., Vol. 33, No. 1 (1978), pages 1 to 3). However, in recent years, demands for display characteristics of a monitor have become severe. Conventionally known anisotropic materials as immobilized in the vertical alignment state are not sufficient with respect to microscopic homogeneity, and improvements thereof have been demanded.